<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>id选择器</title>
    <style>
        #outer{
            color: red;
            font-size: 20px;
        }
    </style>
</head>
<body>
    <!-- id属性值必须要保证唯一性 -->
    <div id="outer">
        outer
    </div>

    <p id="inner">
        p段落
    </p>
</body>
</html>